El Niño and La Niña events are
classified by a number of different criteria. Some classification
systems use the strength and sign of the Southern Oscillation Index (SOI),
while others use Sea Surface Temperature (SST) anomalies for a variety of
Pacific regions. Still others use a combination of several criteria
to gauge the type and strength of the event. Consequently there are
number of different lists which are actively used. Four of the most
widely used lists are:

An event consensus was arrived at by choosing years
which appeared on three of the four above lists. It should be noted
that there is obviously some crossover of the variables used in the
various methodologies and no attempt has been made to give a weight of one
list over another. When WRCC, CPC and MEI all indicated W+ and
CDC gave a W (their strongest category) then that season was considered a
strong event. Otherwise the strength was
determined from the "average" of the strength of chosen events. The
resultant data is expressed in Table 1 (below).